Paper electrostatic masters for lithographic printing, and the methods for making the same, are well known. Lithography depends upon the immiscibility of a greasy lithographic printing ink and an aqueous etch or lithographic solution. In use, a paper lithographic master is first imaged in a known manner and the imaged plate is then placed on a plate cylinder of an offset duplicating press. The overall surface of the plate is treated with an aqueous wet-out or fountain solution which wets all portions of the plate except those areas which have been imaged and are water repellant. The press inking rolls then pass over the surface of the plate and deposit a film of ink only upon the ink-receptive imaged areas. In the printing operation the ink from the imaged areas is transferred in reverse to a rubber offset blanket which in turn prints directly onto a paper sheet so as to form a copy.
Although imaging of the master can be obtained in a number of ways, the present invention is concerned with the preparation of masters suitable for imaging by photoelectrostatic reproduction. This type of reproduction depends upon the presence of a light sensitive photoconductive pigment dispersed in an insulating matrix of a resinous, film-forming material. An electrostatic charge is applied to the surface of the photoelectrostatic coating in the absence of light, and on exposure of the charged surface to an optical image, the charge is dispersed except in those areas which are imaged. Toning of the surface then converts the electrostatic image to a permanent visible image which is ink receptive.
During the printing step, in the offset duplicating press, the fountain solution is compressed or squeezed by the press rolls against the surface of the lithographic plates. This is a more severe exposure to water than most applications for paper and the water is readily worked into the body of the paper. If this occurs, the surface of the plate may become less completely wetted-out by water because water has been withdrawn from the surface into the plate. Therefore, the surface may not repel the printing ink, and areas of the surface which should be perfectly blank will darken or "tone". In addition, absorption of water into the base paper is likely to cause picking of the coats of the electrostatic master, resulting in loss of image in areas where the picking has occurred.
These and other difficulties are particularly great when the plates are used for long runs and are therefore subjected to repeated wettings and inkings. Attempts have been made to overcome the difficulties by employing water resistant barrier coatings beneath the photoconductive layer. For instance, it is known to employ a styrene-butadiene latex, in the amount of about 15% of the barrier coat formulation, with about 5% casein or protein and about 80% filler, primarily coating clay. Other coating materials which have been employed are acrylates, polyvinyl acetates, polyvinylidene chloride, and ethylene vinyl chloride. All of these materials have a common characteristic in that they are all manufactured using surfactants in the polymerization process to form the latices. Surfactants, because of their ionic or hydrophilic nature, tend to reduce water resistance in the polymer film, providing a less than satisfactory water barrier.
For purposes of the present invention, water resistance is defined in terms of the amount of liquid water passing into the sheet in a given time. The test commonly employed to measure water resistance is referred to as the Cobb test, and the amount of water absorbed is measured in terms of grams per square meter of substrate. The period of exposure may be 21/2 minutes or 10 minutes.
It is known to apply formulations to a paper base, which formulations contain ethylene acrylic acid. This is, for instance, disclosed in U.S. Pat. No. 3,541,033 to Buttrick et al, patented Nov. 17, 1970, wherein the ethylene acrylic acid coating is said to provide improved wet rub characteristics in printing paper. The ethylene acrylic acid is applied from a latex formulation containing a surfactant in combination with a large percentage of clay and the resulting coating would provide a less than satisfactory water barrier for electrostatic masters.
It is also known to use ethylene acrylic acid, in either latex form or colloidal solution form, as sizing agents for paper. This is disclosed in U.S. Pat. Nos. 3,239,371 to Whitney et al., (Mar. 8, 1966); 3,899,389, issued Aug. 12, 1975 to Vaughn et al; 3,872,039, issued Mar. 18, 1975, to Vaughn et al; and 3,674,896, issued July 4, 1972 to Purcell et al. These patents teach the use of ethylene acrylic acid both in latex form and colloidal solution form as sizing agents for paper. Either retention aids, waxes, cross-linking polymers, or other polymer additives are required. Sizing refers to the treatment of fibers to provide water repellancy, and is carried out at either the wet end of the paper machine or at a size press of the machine. It is not generally considered a coating procedure in the sense that continuous films are formed on a paper substrate. By way of example, paper sized with ethylene acrylic acid in accordance with the procedure of U.S. Pat. No. 3,674,896 will allow a water absorption, as determined by the Cobb test, of about 62 grams per square meter. By contrast, the water absorption of a paper product, prepared in accordance with the concepts of the present invention, blade coated with ethylene acrylic acid, will have a water absorption on the order of about 15 grams per square meter.
U.S. Pat. No. 3,741,925 teaches the use of ethylene acrylic acid in conjunction with a wax, as a textile size. As with the above patents directed to the preparation of paper products sized with ethylene acrylic acid, this patent is not an anticipation of the present invention, for the same reasons.